Ward

* Exiting during a test or fixture will trigger a failure 165
* Bump `toml` to 0.10.0 164
* Add coverage reporting with Codecov 167

Thanks hoefling and JoshKarpel!

If tests are incorrectly parameterised (e.g. mismatch in `each` argument counts), they will now fail without running and won't be expanded into parameterised instances.

The `--show-diff-symbols` option has been added, which allows you to present diffs using symbols rather than background colours. This improves accessibility and support for CI systems that don't support ANSI terminal output.

<img width="600" alt="Screenshot 2020-05-02 at 16 33 23" src="https://user-images.githubusercontent.com/5740731/80868462-a8016900-8c92-11ea-8263-d8e155f054c6.png">

Workaround for a bug in virtualenv which means site.getsitepackages() is not available inside virtual envs. https://github.com/pypa/virtualenv/issues/228

Ward will no longer look in site-packages when discovering tests, preventing interference from third party packages that don't exclude tests from their distributions.

Thanks to anlutro for this!

Adds the `fixtures` subcommand. By default this command lists out all fixtures present in the project. For example, running `ward fixtures` gives us the following output:

<img width="503" alt="Screenshot 2020-05-01 at 13 10 12" src="https://user-images.githubusercontent.com/5740731/80804451-1b30af80-8bad-11ea-9752-52fa27763b04.png">

We can also run `ward fixtures --show-dependency-tree` to see the relationship between fixtures:

<img width="473" alt="Screenshot 2020-05-01 at 14 44 08" src="https://user-images.githubusercontent.com/5740731/80809747-3a820980-8bba-11ea-9d5b-6e8c39e58081.png">

Thanks to JoshKarpel for this contribution!